The Mob Arena presents players with unending waves of mobs to defeat which grow progressively more difficult over time.
From the Mob Arena lobby, talk to the clown at the ticket booth to join the arena. The cost is one ruby. After paying, you will either be placed into an ongoing game if there is one currently in progress or into an empty arena while the game prepares if there is not. Weapons, armor, and tools do not lose durability while in Mob Arena.
To beat a wave, every mob must be killed. The boss bar at the top of the screen will inform you of your progress. If mobs get lost or stuck, they will begin to glow and be teleported back to their spawn points after a short while.
Between waves, you will have a few seconds of prep time before the next one begins. Use this opportunity to reload crossbows, drink potions, eat food, reposition, or anything else you may find advantageous.
Every tenth wave, a boss chest will spawn near the center of the arena. Open it to select your reward. You will have the option to either upgrade an item already in your inventory or to select a prize. Once everyone has selected a reward or if the wave timer runs out first, you will be teleported to a new arena for the next set of waves.